Do you know how hard it is not to type "feat. Charli XCX" on this?
You aren't imagining things. That's her, on the chorus, uncredited. It's obvious. Some critics have even been critical of it, calling it a Sucker outtake (which may or may not be true).
It's also the rawest song Selena Gomez has ever sung, and it shows real musical maturity. I, for one, am proud of her growth as an artist. The song, as performed, isn't catchy because some hip songwriting Brit decided to allow her backing vocal to remain on the final cut. It's catchy because it's expertly delivered by Selena Gomez.